1. Field of the Invention
This invention relates in general to earth boring drill bits and in particular to an improved lubricant pressure compensator for reducing pressure differential across the cutter seals.
2. Description of the Prior Art
Earth boring drill bits commonly used in oil and gas wells utilize three conical cutters, each mounted on bearings on a depending head section. Lubricant is supplied to the bearings from a reservoir in each head section. A seal at the base of each cutter prevents borehole liquid and cuttings from entering the bearing areas. The seal performs better if there is little or no pressure differential between the lubricant and the borehole fluid.
Pressure compensators have been used in the past to minimize pressure differential across the seals, such as shown in my prior U.S. Pat. No. 4,055,225. The most common type uses a diaphragm that has one side in contact with lubricant and the other side in contact with borehole fluid. As the bit descends in the well, the diaphragm flexes toward the lubricant portion of the reservoir to increase the lubricant pressure in response to the increasing hydrostatic pressure of the borehole fluid.
The temperature in a well increases with depth. During rotation, friction may further increase the temperature of the bit. This temperature increase will cause the grease and trapped gasses in the bit to expand, resulting in a pressure increase in the lubricant reservoir. Movement of the diaphragm toward the borehole fluid will reduce the pressure buildup in the reservoir until the diaphragm reaches its maximum outward position. My prior U.S. Pat. No. 3,942,596 shows a pressure relief valve to be used in conjunction with the diaphragm compensator to allow lubricant to bleed out of the reservoir when the diaphragm is at its maximum position. Also, compensator configuration and positioning, such as shown in my prior U.S. patent application, Ser. No. 885,723, filed Mar. 13, 1978, U.S. Pat. No. 4,276,946, allows the lubricant portion of the reservoir to increase in volume over the original volume at the surface, to prevent excessive pressure increase in the lubricant due to temperature increase.
While the device shown in the above-identified patent application is successful, simplification of structure is desirable, since the diaphragm assembly has several components. Devices simpler in structure, such as sliding pistons, are shown in several patents, such as U.S. Pat. Nos. 3,199,878 and 3,365,247. None of the proposals for sliding pistons, however, have means incorporated with the piston for relieving excess lubricant pressure when the piston is at its maximum outward position.